1. Field of the Invention
The present invention relates to an electrophotographic photoreceptor (also referred to as simply a “photoreceptor” below) and a process for producing such an electrophotographic photoreceptor. More specifically, the invention relates to an electrophotographic photoreceptor which is made primarily of a conductive substrate and a photosensitive layer containing an organic material, and which is used in, for example, electrophotographic printers, copiers and facsimile machines, and relates also to a process for producing such an electrophotographic photoreceptor.
2. Background of the Related Art
Image-forming methods that employ electrophotographic technology are used widely not only in office copiers, printers and plotters, and in digital multifunctional devices which combine the functions of these pieces of equipment, but also more recently in small printers and facsimile machines for personal use. Ever since Carlson's invention (U.S. Pat. No. 2,297,691 (Patent Document 1) of a photoreceptor for an electrophotographic device, many photoreceptors have been developed, and photoreceptors which use organic materials have become particularly commonplace.
Such photoreceptors include multi layer photoreceptors composed of a conductive substrate such as aluminum having stacked thereon: an undercoat layer such as an anodic oxide film or a resin film; a charge generation layer containing an organic pigment having photoconductivity, such as a phthalocyanine or azo pigment; a charge transport layer containing molecules having a partial structure which participates in charge hopping conduction, such as an amine or hydrazine coupled to a π-electron conjugated system; and a protective layer. Single layer type photoreceptors which combine the charge generation and charge transport functions are also known. A single layer type photoreceptor may also have an undercoat layer.
Each of these layers is generally formed by a method that involves dip-coating the conductive substrate in a coating obtained by dissolving or dispersing a pigment having charge-generating, light-scattering and other functions or a charge transport material which assumes the role of transporting charges in a suitable resin solvent. Such a method is used because it has an excellent mass productivity.
Recent electrophotographic devices employ predominantly the so-called reversal development process which uses a semiconductor laser having an emission wavelength of about 450 to 780 nm or light-emitting diodes as the exposure light source to convert digital signals for images, text and the like into light signals, shines the light signals onto an electrically charged photoreceptor so as to form an electrostatic latent image on the surface of the photoreceptor, and renders the latent image visible by means of a toner.
Moreover, in such electrophotographic devices, phthalocyanines, which have a large light absorbance in the semiconductor laser emission wavelength region compared with other charge generation materials and also have an excellent charge generating ability, are being widely studied as photosensitive layer materials. In particular, photoreceptors which use various types of phthalocyanines having aluminum, indium, vanadium, titanium, etc. as the center metal are known today (Japanese Patent Application Laid-open No. S53-89433 (Patent Document 2), U.S. Pat. No. 3,816,118 (Patent Document 3), Japanese Patent Application Laid-open No. S57-148745 (Patent Document 4), and U.S. Pat. No. 3,825,422 (Patent Document 5).
Methods for electrically charging photoreceptors include a non-contact charging system in which a charging member that operates such as by corona discharge from a scorotron remains in a non-contact state with the photoreceptor, and a contact charging system in which a charging member in the form of a roller composed of conductive rubber or a brush composed of conductive fibers comes into contact with the photoreceptor. Compared with a non-contact charging system, such a contact charging system has a short discharge distance in open air. As a result, it generates little ozone, the power supply voltage may be low, and the system is maintenance-free because there is no deposition on the charging member of scum that arises due to discharge. Moreover, such a contact charging system is able to keep the charge potential on the photoreceptor uniform. Accordingly, medium and small-size electrophotographic devices predominantly use contact charging systems because such a system makes it possible to achieve devices which are more compact, lower cost, and less environmentally polluting.
However, it is known from experimental results on many compounds that, when a charge generation layer and a charge transport layer are combined to form a photosensitive layer, very few compounds satisfy the photoreceptor characteristics and conditions required for practical use. In particular, few compounds satisfy the repeated charging and exposure characteristics of known electrophotographic processes; when repeated charging and exposure is carried out, this invites a rise in residual potential that is believed to be caused by charge trapping and accumulation in the charge transport layer. Owing to this influence, changes in image density due to repetition arise in the reversal development process employed in printers and the like. Such changes are presumed to be due to light-induced fatigue. This problem similarly arises also in single layer type photoreceptors obtained by dispersing the above phthalocyanine pigment, a bisazo pigment or the like in a resin binder, and applying the resulting dispersion.
The addition of specific additives to a charge transport layer in order to prevent photodeterioration is known. For example, Japanese Patent Application Laid-open No. H10-228121 (Patent Document 6) discloses art for adding an orange dye compound, Japanese Examined Patent Publication No. H8-33660 (Patent Document 7) discloses art for adding an orange colorant, and Japanese Patent Application Laid-open No. H11-184108 (Patent Document 8) discloses art for adding a compound having a maximum absorption wavelength at 480 nm.
Also, to prevent a decline in electrostatic properties due to exposure-induced fatigue, Japanese Patent Application Laid-open No. S60-104951 (Patent Document 9), Japanese Patent Application Laid-open No. H2-264263 (Patent Document 10), Japanese Patent Application Laid-open No. H5-112508 (Patent Document 11), and Japanese Patent Application Laid-open No. H7-120950 (Patent Document 12) disclose, as charge transport materials, ethylene compounds having many double bonds.
However, as mentioned in Patent Documents 6 to 8 and elsewhere, the art for adding specific additives to a charge transport layer is limited to phthalocyanine pigments or specific charge transport materials. These documents describe nothing other than photodeterioration-preventing effects for preventing photodegradation of the charge transport material, etc. by taking into account light absorption. The compounds mentioned in Patent Documents 9 to 12 did not exhibit sufficient effects for light-induced fatigue.
It is therefore an object of the present invention to provide an ethylene compound and a charge transport material which have a low photodeterioration. Further objects of the invention are to provide an electrophotographic photoreceptor which has a low light-induced fatigue and is capable of preventing the rise in residual potential that accompanies fatigue; an electrophotographic photoreceptor which, owing to these characteristics, has stable properties as an electrophotographic photoreceptor even when used for an extended period of time, and can stably obtain satisfactory images; and a process for producing such an electrophotographic photoreceptor.